Dean's never met someone as in love with their car as he is. Everyone knows how much he loves Baby, how protective he is over the car. Not a single person is allowed to drive Baby, or slander her in way shape or form. Even suggesting that another car is better than Baby is complete treason, and has Dean disliking you a little more because of it. That's where you come in.
Dean's relationship with Baby is the same as you and your car. Your car is your pride and joy, you care for it as if it's a whole person, and don't tolerate any disparaging comments towards the vehicle. As does Dean. The issue is, you think your car is better than Baby. That shit does not fly with Dean—how can any car be better than his Baby? It sounds literally insane to him, to be completely honest. And to think that any car could be better than your own is equally as insane to you.
See the dilemma? It was so fucking annoying for everyone involved, the bickering and arguing between the two of you about cars. No one could get in a word anyway, with the two of you most likely snapping at whoever tried to tell you you're overreacting, no matter whether it was Sam, Cas, fuckin' Crowley—it was a feud between you two, and the two cars.
The amount of times you'd shit talked Baby infront of Dean, only for him to gasp incredulously and tell the car—"Don't listen to them, they're just jealous," was getting out of control. And no, the two of you would never race because those cars would sure as hell never see the light of day again after the chaos that would insue as a result.
You argue like a damn married couple. "I don't know who convinced you that hunk of metal is as great as you make it out to be—but it ain't," he snarks, starting off another one of your stupid arguments when you'd mentioned that you'd gotten a compliment on your car.
Here it went again.